A. Field of the Invention
The invention relates to a spline hub and a method of manufacturing the same, and in particular the invention relates to a spline hub provided at an inner peripheral surface thereof with a circumferential lubricant-filled groove, i.e., a circumferential groove to be filled with lubricant.
B. Description of the Background Art
A clutch disk assembly used in an automobile is formed with a clutch plate and a retaining plate that together define input members, a spline hub disposed at a central portion of the plates, the spline hub defining an output member, and a damper mechanism between the plates and hub for dampening vibrations. The damper mechanism includes coil springs and other members arranged between the spline hub and the clutch and retaining plates. A clutch disk having at least one cushioning plate and friction facings is fixed to outer peripheral portions of the clutch and retaining plates.
The spline hub is formed with a cylindrical boss having a central bore. The central bore is provided with a plurality of spline grooves engageable with a shaft extending from a transmission. A flange extends radially outward from the boss. The flange is provided with windows or openings that accommodate coil springs. A clutch disk assembly of a separated hub type is also known. In this type, the flange is formed as a separate member from the boss and encircles the boss. The flange is coupled to the boss in a rotating direction via elastic members having a rigidity that is lower than the coil springs that couple the flange and the plates.
In the prior art, the spline hub is manufactured in a series of main steps, i.e., forging, turning, pressing, refining and broaching. In the forging step, a rod member is subjected to cutting, heating, shaping and trimming to prepare a basic form of the spline hub. In the turning step, turning is effected on the opposite side surfaces of the flange as well as the outer peripheral surface, end surfaces and inner peripheral surface of the boss. In the pressing step, apertures such as round apertures are formed by the press working, and distortion is removed in a restriking step. In the refining step, quenching and tempering are effected on the spline hub to provide a hardness of HRc 20-30. In an induction quenching step, induction heating is performed by induction coils arranged in the respective windows of the spline hub. Further, tempering at 180.degree. C. is performed for two hours. In the final broaching step, the spline grooves are formed in the central bore of the spline hub by a broaching machine.
In the conventional spline hub described above, a plurality of grease grooves extend circumferentially along the inner peripheral surface of the central bore of the boss. These grease grooves are typically filled with grease. The grease reduces friction between the shaft of the transmission and the spline grooves of the spline hub when the spline hub slides axially on the shaft.
The end surfaces at the axially opposite ends of each grease groove are substantially perpendicular to the rotation axis of the hub. The grease grooves are formed before forming the spline grooves by the broaching. More specifically, the spline grooves are formed by moving a broach cutter through the central bore which is already provided with the grease grooves. Burrs are left around the grease grooves when forming the spline grooves by the broach cutter. Therefore, working is required for removing the burrs after the machining with the broach cutter. This increases the number of operation steps.